Layer lattice structured dry lubricant coating method

ABSTRACT

Method of applying a dry lubricant coating to a metal substrate by air impingement in the absence of a vehicle or binder including pretreating the particulate dry lubricant by heating at elevated temperature in a vacuum.

United States Patent Nelson LAYER LATTICE STRUCTURED DRY LUBRICANT COATING METHOD Appl. No.: 878,257

Related US. Application Data Continuation-impart of Ser. No. 777,161, Nov. 19, 1968, abandoned, Continuation-impart of Ser. No. 656,028, July 26, 1967, abandoned.

US. Cl. ..117/31, 75/.5 B, 75/.5 BB, 75/6,117/16,117/22,1l7/66,l17/105,117/l30 R, 252/25 Int. Cl. ..B44d 1/094, C23c 7/00 FieldofSearch ..117/66,16,22,31,105,105.1,

117/130, 169; 252/12, 25, 49.7; 75/.5 B, .5 BB, 6

[ Feb.22, 1972 References Cited UNITED STATES PATENTS 2,622,993 12/1952 McCullough et al. ..117/49 2,994,654 8/1961 Fahnoe et a1. ..117/22 3,100,724 8/ 1963 Rocheville 1 18/308 3,101,252 8/1963 Tschudi et a1. .....252/25 3,160,952 12/1964 Corney et a1. 117/105 3,170,785 2/1965 Phillips 75/.5 BB 3,177,067 4/1965 Nichols 75/.5 BB 3,342,636 9/1967 Pakswer ..117/16 3,390,080 6/1968 Groszek ..252/25 3,473,943 10/1969 Kai ..117/16 Primary Examiner-William D. Martin Assistant Examiner--Raymond M. Speer Attorney-Fitch, Even, Tabin & Luedeka [57] ABSTRACT Method of applying a dry lubricant coating to a metal substrate by air impingement in the absence of a vehicle or binder including pretreating the particulate dry lubricant by heating at elevated temperature in a vacuum.

10 Claims, No Drawings LAYER LATTICE STRUCTURED DRY LUBRICANT COATING METHOD This invention relates generally to the preparation of layer lattice structured materials for use as solid lubricants, and more particularly, it relates to the treatment of layer lattice structured materials preparatory to their use in an air impingement process for the application of such materials as solid lubricants. This application is a continuation-in-part of copending Ser. No. 777,161, filed Nov. 19, 1968, now abandoned, and of copending application Ser. No. 656,028, filed July 26, 1967, now abandoned.

There is disclosed in Ser. No. 777,161 a unique and advantageous method of applying an integral adherent continuous coating of a layer lattice structured dry lubricant material to a metal surface. The term dry lubricant, as used herein, is intended to include those materials known generally as layer lattice structured materials. Layer lattice structured materials are generally recognized in the dry lubricant art as materials in which the crystalline lattice structure of the materials is a platlet form in which the metal atoms are centrally located within the platlet and the nonmetallic atoms are arranged on each side thereof. Examples of layer lattice structured materials which are useful as dry lubricants include a number of the chalcogenides and dichalcogenides of the group Vb and Vlb metals, i.e., niobium, molybdenum and tungsten. The disulfides, ditellurides and disellinides of these metals are generally most commonly employed as dry lubricants.

Tungsten disulfide is a preferred form of dry lubricant and the present invention is described in connection with the treatment of tungsten disulfide. It is to be understood, however, that other layer lattice structured dry lubricant materials may also be treated in accordance with the present invention. The method disclosed in Ser. No. 777,161 includes suspending a particulate dry lubricant material in an airstream and impinging the particles against the metal surface at high velocity in the absence of any binder or vehicle. The process as described in Ser. No. 777,161 provides a uniform adherent dry lubricant coating which is diffusion bonded to the surface of the metal. Because of the chemical structure of tungsten disulfide and other layer lattice structured materials, these materials are not capable of bonding to one another upon impingement and the diffusion bonded dry lubricant coating is quite thin, typically 0.000003 to 0.000010 inch in thickness.

In a typical impingement process for the application of tungsten disulfide dry lubricant to a metal surface in the absence of a binder or vehicle, the tungsten disulfide is placed in a hopper within a coating booth. Air pressure is utilized to siphon the tungsten disulfide powder into the jet of an air gun. This form of apparatus is generally similar to that commonly employed in sand blasting. The excess tungsten disulfide which does not become bonded to the surface of the article falls to the bottom of the coating booth and is returned to the hopper for reuse.

Tungsten disulfide which is suitable for use as a solid lubricant is available from various commercial sources. Tungsten disulfide powder for use in an impingement process in the absence of a binder or vehicle desirably has an average particle size of less than about microns and acceptable grades of tungsten disulfide generally have an average particle size between about 1 micron and about microns.

Commercially available tungsten disulfide, in the form obtained from the manufacturer, is generally unsuitable for use in an air impingement coating process in the absence of a binder or vehicle. Commercially available tungsten disulfide has a dark blackish appearance and the individual particles are capable of being compacted and agglomerating to themselves under slight pressures. For example, much of the commercially available tungsten disulfide can be hand packed into balls similar to the way snow can be packed into a snowball. This is generally undesirable and results in a nonuniform lubricant coating on the surface of the article to be coated. in many instances, actual agglomerates may form on the surface. in addition, the agglomerates of the powder may be formed in the hopper of the coating apparatus which can cause clogging and sticking of the powdered materials in the air gun.

Application of a layer lattice structured dry lubricant in an air impingement process without treatment of the powder as described herein may also result in an insufficient diffusion bonding between the lubricant particles and the metal surface being coated. Under conditions of stress or load, such lubricant coating may be removed from the metal surface.

The commercially available sources of tungsten disulfide are further undesirable because they contain excessive amounts of free sulfur which can combine with oxygen and moisture in the ambient air and form sulfuric acid. The formation of sulfuric acid actually impedes the coating operation and may enhance corrosion of the surface to which the lubricant coating is applied.

It is an object of the invention to provide an improved process for the preparation of layer lattice structured dry lubricant materials in order that the such materials may be conveniently used in an air impingement coating process to provide a lubricant coating on the surface of a substrate in the absence of a vehicle or binder. A further object is to provide a method for the application of a coating of a layer lattice struc tured dry lubricant to the surface of a metal substrate which includes a particular preparation of the lubricant powder.

These and other objects of the invention may be readily understood from the following detailed description.

Very generally, the present invention is directed to a method of treating a powdered layer lattice structured dry lubricant, for example, tungsten disulfide, in order to permit the use of such powdered lubricant in an air impingement process for applying the powdered lubricant to a metal surface as a solid lubricant in the absence of a vehicle or binder. The process of treating the powdered lubricant includes providing a particulate layer lattice structured dry lubricant powder having an average particle size of less than about 15 microns, preferably less than about 10 microns, and heating the lubricant powder under vacuum and in the presence of hydrogen for a period sufficient to destroy the tendency of the powdered lubricant to agglomerate.

The process may be carried out by placing the powdered lubricant in a vacuum oven and reducing the pressure in the vacuum oven to about 3 torr. The powdered lubricant is heated in vacuum to a temperature of between about 285 and about 315 F., preferably about 300 F. and while being heated under vacuum, a hydrogen gas stream is introduced into the vacuum oven. The hydrogen gas flow and the vacuum pump used to evacuate the oven are regulated so as to maintain the pressure in the oven below about 10 torr, preferably below about 5 torr.

The heating of the powdered lubricant and the flow of hydrogen gas through the oven is continued until the free or uncombined nonmetallic content of the powdered lubricant is reduced to below about 1.5 percent by weight, preferably below 1 percent by weight, and the moisture content is reduced to below about 05, preferably below about 0.25, percent by weight. The thusly treated powdered lubricant is then cooled to room temperature in the absence of reactive gases, e.g., oxygen, and moisture. Preferably, the cooling of the powdered lubricant is affected with a positive hydrogen pressure in the furnace, for example between about 2 and about 5 p.s.i.g. hydrogen.

It has been discovered that the treatment of a powdered lubricant in the manner set forth herein provides a surprising change in the physical appearance of the powdered lubricant and in its ability to be utilized in an air impingement coating process in the absence of a vehicle or binder. The powdered lubricant prior to treatment is generally dark in color and when compressed, as by squeezing with the hand, readily agglomerates and forms a ball or lump in much the same fashion as snow. The powders subsequent to treatment are light and fluffy and have a somewhat lighter color. In addition, after treatment in accordance with that described herein, the powders cannot be agglomerated or compacted with the hands 101024 OISS and in general are not cohesive to one another under any normally encountered condition.

There is a surprising and measurable difference in the ability of the powdered lubricant to be utilized in an air impingement coating process in the absence of a vehicle or binder after treatment as described herein. The treated powdered lubricant has no tendency to agglomerate. Therefore, the possibility of clogging of the air lines is eliminated. Furthermore, experience in coating with tungsten disulfide in an air impingement coating process in the absence of a vehicle or binder has repeatedly demonstrated that lubricant powders which have been treated in accordance with the disclosed process are capable of forming a continuous lubricant coating on the surface of a metal substrate in a much shorter period of time than similar powdered lubricants which have not been treated in accordance with the disclosed process. The use of treated powders also results in a more uniform coating on the substrate than do untreated powders. Further, there is no tendency to form agglomerates in the spray gun or on the surface of the substrate when treated powders are used in an air impingement coating process in the absence of a vehicle or binder, while use of untreated powders in a similar process generally results in agglomerates forming on the surface of the substrate and also in the clogging of the spray gun.

As indicated, the described process has particular application in the treatment of tungsten disulfide powder. Commercially available tungsten disulfide powder has a particle size of generally about one-half micron and about 10 microns, with most of the particles having a particle size in the range of l to microns.

The tungsten disulfide powder may be treated in any suitable vacuum oven, and various styles and sizes of vacuum ovens are commercially available. It is desirable to expose as large a surface area of the tungsten disulfide powder to the vacuum and hydrogen gas as possible and in this connection the powder is desirably spread in layers having a thickness of between about 1% and about 2% inches on several trays which are placed on racks in the oven. The oven is sealed and evacuated to a low pressure of between about 3 and about torr, usually about 5 torr. When the oven has been evacuated, it is heated for example by means of radiant or resistance heating, to a temperature above 285 F. and generally between about 290 and about 305 F. and preferably about 300 F. lt has been found that added advantages do not appear to be obtained by heating the tungsten disulfide powders to much above 310 F., but it is contemplated that the dry lubricant powder can be heated to a higher temperature up to temperatures where the dry lubricant decomposes or reacts with its surroundings. it is also possible to initiate the heating during the time the oven is being pumped down to the desired vacuum.

A purge stream of hydrogen gas is then introduced into the vacuum oven while maintaining the temperature at about 300 F. and continuing to evacuate the oven to maintain 'a low pressure therein of about 10 torr. The flow rate of hydrogen gas is adjusted in accordance with the capacity of the vacuum pump. Generally, it has been found that a hydrogen flow rate of between about 2 and about 3 cubic feet per hour is sufficient to provide the desired purging of the system, but it is obvious that other flow rates may be utilized if desired. The hydrogen gas which is contacted with the hot tungsten disulfide reacts with the free sulfur, i.e., the uncombined nonmetallic component, that is present in the tungsten disulfide to form hydrogen sulfide gas which is exhausted from the oven. ln addition, the purging stream of hydrogen causes adsorbed gases such as oxygen, nitrogen and water vapor to be liberated from the tungsten disulfide powder.

The purge stream of hydrogen gas is contacted with the tungsten disulfide powder for a period of time sufficient to reduce the free sulfur content of the tungsten disulfide to less than 2 percent by weight, and preferably less than about 1 percent by weight. This period may be as short as 2 hours or as long as 10 hours. The length of time of contacting of the tungsten disulfide with hydrogen is, of course, dependent upon the initial free sulfur content of the tungsten disulfide, the size of the furnace and the amount of tungsten disulfide treated at a time. Generally, when the tungsten disulfide contains about 1.2 to 1.5 percent free sulfur, and the vacuum oven contains about 30 pounds tungsten disulfide with the hydrogen flow rate of about 2 to 3 cubic feet per hour, the free sulfur content will be reduced to below about 1 percent by weight in about 4 hours.

In addition to removing free sulfur from the tungsten disulfide, the heating under vacuum and purging with hydrogen also removes moisture from the tungsten disulfide. Moisture is not desirable in the tungsten disulfide because it tends to cause agglomeration, and because it can react with the free sulfur to form sulfuric acid, which, of course, is not desirable where a metal surface is to be coated.

The hydrogen gas flow is continued until the tungsten disulfide has a desired purity. Subsequently the tungsten disulfide is cooled to ambient conditions in the absence of reactive gases such as oxygen and water vapor or moisture. The tungsten disulfide can be cooled by turning off the hydrogen flow and cooling the tungsten disulfide in the vacuum. However, because of the long times required to cool hot materials in vacuum it is generally preferably to turn off the vacuum pump and continue the flow of hydrogen into the furnace until the pressure within the furnace reaches a positive pressure, for example, 2 to 5 p.s.i.g. Higher pressures than this are not necessary, but can be used if desired. The introduction of hydrogen gas into the furnace until a positive pressure is achieved, and cooling of the particles of tungsten disulfide in the presence of hydrogen, ensures that there is an absence of undesired free sulfur and moisture in the tungsten disulfide particles. There is also an added advantage in that the pores of the particles and the interstitial spaces between the particles in the batch of tungsten disulfide become permeated with hydrogen. This is of an advantage when packaging the treated powder to insure that there is a nonreactive atmosphere surrounding the particles during storage.

Upon cooling the particles of tungsten disulfide powder are packaged in a suitable container. Generally, it is most preferable to flush the container, for example, a paint can or other water impervious container, with dry hydrogen, dry nitrogen, dry argon, or any dry inert gas prior to introduction of the tungsten disulfide powder. As previously indicated, the presence of hydrogen in the powder because of cooling under a positive hydrogen pressure also aids in preventing the subsequent contamination of the powder by high moisture levels.

It will be seen that a particular treatment method for treating layer lattice structured dry lubricant materials which are to be utilized in an air impingement coating operation in the absence of a vehicle or hinder has been disclosed. The treated lubricant powder provides advantages in the coating operation through ease of application and the freedom from forming agglomerates either in the coating booth or on the surface of the coated article. The treated lubricant powder also offers the advantage of being more readily bonded to the substrate surface than is the untreated lubricant powder.

It is to be understood that the disclosed process has been set forth with particularity in order to accurately describe the steps in the process. Various alternative measures, considered to be within the skill of the art are contemplated.

Various of the features of the invention are set forth in the following claims.

What is claimed is:

1. ln an impingement process for applying a dry lubricant coating on a metal substrate wherein a particulate layer lattice structured dry lubricant material of the disulfides, ditellurides or disellinides of the group Vb and Vlb metals, having an average particle size of less than about 15 microns is entrained in a gaseous stream and impinged against the metal substrate in the absence of any vehicle or binder and with sufficient velocity to cause the dry lubricant to become diffusion bonded to the substrate, the improvement which comprises prior to use in the process, heating the dry lubricant material to a temperature above about 285 F. in a vacuum environment of below about torr, contacting the dry lubricant with hydrogen gas while maintaining the dry lubricant at a temperature above about 285 F. and at a pressure less than about 10 torr until the amount of any uncombined nonmetallic content of the dry lubricant is reduced to below about 1.5 percent by weight and the moisture content of the dry lubricant is reduced to below about 0.5 percent by weight, and cooling the dry lubricant in the absence of reactive gases and moisture.

2. A process in accordance with claim 1 wherein the dry lubricant is heated at a pressure below about 3 torr.

3. A process in accordance with claim 2 wherein contact between the hydrogen gas and the dry lubricant is continued until the uncombined nonmetallic content of the dry lubricant is below about 1 percent by weight and the moisture content of the dry lubricant is below about 0.25 percent by weight.

4. A process in accordance with claim 3 wherein the hydrogen gas is continuously introduced and withdrawn from the vacuum environment containing the dry lubricant.

5. A process in accordance with claim 4 wherein the hydrogen is introduced into the vacuum environment at a flow rate of about 2 to 3 cubic feet per hour.

6. A process in accordance with claim 4 wherein the flow of hydrogen through the vacuum environment is continued for between about 2 and about 10 hours.

7. A process in accordance with claim 3 wherein the dry lubricant is cooled in a hydrogen atmosphere.

8. A process in accordance with claim 6 wherein the dry lubricant is cooled in a hydrogen atmosphere at a pressure above about 2 p.s.i.g.

9. A process in accordance with claim 1 wherein the dry lubricant is tungsten disulfide.

10. A process in accordance with claim 6 wherein the dry lubricant is tungsten disulfide. 

2. A process in accordance with claim 1 wherein the dry lubricant is heated at a pressure below about 3 torr.
 3. A process in accordance with claim 2 wherein contact between the hydrogen gas and the dry lubricant is continued until the uncombined nonmetallic content of the dry lubricant is below about 1 percent by weight and the moisture content of the dry lubricant is below about 0.25 percent by weight.
 4. A process in accordance with claim 3 wherein the hydrogen gas is continuously introduced and withdrawn from the vacuum environment containing the dry lubricant.
 5. A process in accordance with claim 4 wherein the hydrogen is introduced into the vacuum environment at a flow Rate of about 2 to 3 cubic feet per hour.
 6. A process in accordance with claim 4 wherein the flow of hydrogen through the vacuum environment is continued for between about 2 and about 10 hours.
 7. A process in accordance with claim 3 wherein the dry lubricant is cooled in a hydrogen atmosphere.
 8. A process in accordance with claim 6 wherein the dry lubricant is cooled in a hydrogen atmosphere at a pressure above about 2 p.s.i.g.
 9. A process in accordance with claim 1 wherein the dry lubricant is tungsten disulfide.
 10. A process in accordance with claim 6 wherein the dry lubricant is tungsten disulfide. 